Multi-user communication (multiplex communication) between a base station (hereinafter, which may be referred to as an access point) and a plurality wireless communication terminals (hereinafter, which may be referred to as terminals or terminals) is discussed. Uplink multi-user communication is represented as UL-MU (UpLink Multi-User) communication, and downlink multi-user communication is represented as DL-MU (DownLink Multi-User) communication.
As the multi-user communication, frequency multiplexing communication is known according to which different frequency components for each terminal are used as communication resources and transmissions to a plurality of terminals and receptions from a plurality of terminals are simultaneously performed. Here, Orthogonal Frequency Division Multiple Access (OFDMA) scheme is considered where the frequency components are defined as resource units each including one or a plurality of subcarriers, and the resource units each are used as a smallest unit of the communication resource, and transmissions to the plurality of terminals or receptions from the plurality of terminals are simultaneously performed. The simultaneous transmissions from the access point to the plurality of terminals correspond to downlink OFDMA (DL-OFDMA) transmission and the simultaneous transmissions from the plurality of terminals to the access point correspond to uplink OFDMA (UL-OFDMA) transmission. The DL-OFDMA is an example of the DL-MU, and The UL-OFDMA is an example of the UL-MU.
In a case of the UL-OFDMA, it may be considered that in order to match timings of uplink transmission, a trigger frame specifying terminals that are to be subjected to the UL-OFDMA and resource units allocated to the terminals is transmitted from an access point. This method has problems that in a case such as where the specified terminal is transited to a sleep mode, or the specified terminal has no uplink transmission request, the resource unit allocated to the specified terminal is not efficiently used and a usage efficiency of a communication resource is decreased.
There is another method in which the trigger frame does not specify any terminal, but specifies only resource units that are to be used. A terminal having received a trigger frame randomly selects a resource unit from among resource units without terminal specification (sometimes referred to as “STA-unspecified RU”), and transmits a frame serving as a response frame. Such a trigger frame containing specification of STA-unspecified RU is sometimes referred to as a trigger frame for random access (TF-R). In some cases, there is a TF-R in which only some resource units are STA-unspecified RUs, and the other resource units have terminal specification.
Methods of causing a terminal having received TF-R randomly to select a resource unit from among STA-unspecified RUs include the following method. A random number (backoff value) selected from a contention window (CW) for random access is decremented by a value according to the number of STA-unspecified RUs every time of receiving TF-R. The backoff value is managed on a terminal-by-terminal basis. When a STA-unspecified RU having a determined backoff value equal to or less than zero occurs, the terminal obtains an access right, randomly selects a resource unit from among the STA-unspecified RUs specified by TF-R, and transmits a response frame in the selected resource unit predetermined time (xIFS) after completion of receiving TF-R. Such random selection of the resource unit can increase the possibility of avoiding frame collision at an access point at in a case of access between multiple terminals. Note that a CW for random access is different from a contention window used to determine the backoff time at the time of CSMA/CA carrier sense.
TF-R can be used to collect an UL-MU allocation request from a terminal (uplink transmission request). The access point receives response frames containing uplink transmission requests from multiple terminals in response to the transmitted TF-R (UL-OFDMA based on TF-R). The access point allocates, to the terminal, the same resource unit as the resource unit in which the response frame has been transmitted. The resource units to be allocated to the multiple terminals are determined through one or more times of TF-R transmission, and subsequently, a trigger frame that specifies the multiple terminals and resource units allocated to the terminals is transmitted. The terminals that have received the trigger frame and been designated by the trigger frame transmit a data frame containing data for uplink transmission using the specified resource unit the predetermined time (xIFS) after the trigger frame (UL-OFDMA based on the trigger frame), respectively.
The communication qualities of the terminals on a resource unit basis typically vary; however, this feature depends on the bandwidth of the resource unit and the channel to which the resource unit belongs. The communication quality may be, for example, SNR (Signal to Noise Ratio), RSSI (Received Signal Strength Indicator), reception EVM (Error Vector Magnitude) or the like. According to the communication quality, MCS (Modulation and Coding Scheme) usable by the terminals varies. To improve the UL-OFDMA efficiency based on the trigger frame, it is preferred to allocate resource units with communication qualities so as to allow a high MCS to be applied to the terminals. However, allocating resource units randomly selected by the terminals to the terminals cannot achieve appropriate resource unit allocation, which causes a possibility of reducing UL-OFDMA efficiency.